1. Technical Field
This invention relates generally to the field of optoelectronic device manufacturing and, more specifically, to encapsulants for optoelectronic devices, such as vertical cavity surface emitting laser (VCSEL) devices.
2. Background Art
Development of improved fiber optic data interconnects has been a goal within the optoelectronics industry. The need for increasing the rate at which information is passed between computers or between a computer and a peripheral device has been steadily rising. In the past, high speed interconnection has been accomplished to a certain extent with copper wire cables. These cables often include multiple wires with the wires being single conductors, bundled or in a ribbon configuration, coaxial cables, etc. and may be shielded to reduce electromagnetic interference (EMI) in a variety of ways.
Nevertheless, several disadvantages exist with current high speed copper wire cables that are driving a transition away from copper wires to fiber optic technology. First, it is desirable to provide a large number of parallel conductors within a cable for certain applications, but the size of cables and connectors in copper wire technology are becoming unacceptably large. Second, copper wire cables radiate radio frequency power and the radiation can be difficult to control during high speed data transmission. Further, such cables are also susceptible to picking up radio frequency signals from external noise sources that are difficult to minimize. Third, high speed copper wire cables are rather costly, thus, providing a cable of sufficient length for applications currently contemplated can be cost prohibitive.
Parallel fiber optic data interconnects offer the potential of providing low cost, compact, low EMI, high speed data transmission over suitable distances. Typically, a parallel optoelectronic package includes a receiver and a transmitter interfaced with separate optical fibers even though a single optical fiber has the potential to provide simultaneous, bidirectional communication. The receiver further includes a semiconductor die with light detectors such that light signals may be received through optical fiber and converted to electrical signals for use by the receiving computer or peripheral. A transmitter is similar except that it includes a semiconductor die with light sources such that electrical signals from the transmitting computer or peripheral may be converted to a light signal and transmitted through optical fiber. A variety of light emitting diodes (LEDs) and lasers are used as light sources, however, one particularly promising light source is a vertical cavity surface emitting laser (VCSEL). Multiple patents and technical publications exist which explain VCSEL technology and its potential uses in fiber optic data interconnects. Nevertheless, challenges have been encountered with incorporating VCSEL technology into parallel optoelectronic packages.
Parallel optoelectronic packages require alignment of a VCSEL array, or another array of light sources, and a light detector array to optical couplers. The optical coupler of a transmitter receives light generated by the VCSEL array and channels light from each VCSEL to an individual optical fiber in a transmission cable. Similarly, the optical coupler of a receiver channels light from individual optical fibers in a transmission cable to each light detector in the light detector array. One suitable configuration for a transmitter consists of a semiconductor die with a VCSEL array bonded to a gold/nickel-plated metal carrier to dissipate thermal energy generated by the lasers. Depending on the nature of the VCSEL array, the semi conductor lasers often require a passivation layer, such as silicon nitride, to provide environmental protection. Precise alignment of the VCSEL array to the optical coupler is critical for adequate performance of the device. Alignment is often accomplished by means of precision stages driven by an appropriate alignment algorithm, such as available from National Instruments in Austin, Tex. Once alignment is complete the optical coupler is secured to the die carrier in any of a variety of manners known to those skilled in the art.
Process considerations dictate that a gap exist between the VCSEL array and optical coupler, often amounting to about 100 micrometers. Such a gap is sufficiently large to allow extraneous debris or subsequent process chemicals to enter the gap, resulting in interfering accumulations of matter forming in the gap. It is a disadvantage that a gap exists between the VCSEL array and optical coupler since degradation of light signals occurs as a result of accumulations in the gap. Accordingly, it would be an improvement in the art to provide an optoelectronic package that functions as desired, yet does not produce signal degradation in the manner described.
According to the present invention, an optoelectronic package is provided including a substrate having an array of one or more sources or detectors for generating or receiving a light signal, respectively, an optical coupler aligned with the array, and an optically transparent substrate encapsulant in contact with the substrate that substantially fills a gap between the substrate and optical coupler. The encapsulant provides the advantage of both passivating the sources or detectors and preventing interfering accumulations of matter from forming in the gap without degradation of the light signal. By way of example, the encapsulant may exhibit optical transparency to light having a wavelength of about 850 nanometers with substantially no Mie scattering. Further, the encapsulant is particularly suitable in an optoelectronic package wherein the sources are vertical cavity surface committing lasers (VCSELs). The encapsulant may also provide the advantage of further sealing wire bonds from the substrate to a cable header to avoid abrasive wear.
An encapsulant is also provided which includes, prior to curing, a functionalized resin and an effective amount of a curing initiator, wherein the encapsulant is adapted to passivate an optoelectronic source. Examples of a suitable encapsulants exhibit pseudoplastic flow prior to curing and may also exhibit a viscosity of greater than about 0.7xc3x97106 centipoise prior to curing. The functionalized resin may be a difunctional acrylated epoxy resin.
The present invention also involves a method of manufacturing an optical subassembly including the steps of: affixing a non-passivated die with an array of sources or detectors to a die carrier, aligning an optical coupler with the array, dispensing an encapsulant on at least the array, and curing the encapsulant. Additionally, the cured encapsulant may substantially fill a gap between the die and optical coupler to prevent interfering accumulations of matter from forming in the gap. Also, the step of dispensing may include further dispensing the encapsulant on wire bonds from the die to a cable header.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.